U.S. Pat. No. 4,218,543 describes the production of reaction injection molded (RIM) products using reaction mixtures of diisocyanates or polyisocyanates based on 4,4'-diisocyanatodiphenyl methane, aromatic polyamines, and relatively high molecular weight polyhydroxyl compounds.
Recent advances in the RIM area have been directed to the substitution of so-called "amino-polyethers" for all or a portion of the relatively high molecular weight polyhydroxyl compounds. See, e.g. U.S. Pat. Nos. 4,774,263, 4,396,729, 4,433,067, 4,444,910, 4,530,941 and 4,590,219; and German Offenlegungsschriften Nos. 3,215,907 and 3,215,909. Typically, the isocyanates used with the amino-polyethers have included (i) liquid products made by reacting 4,4'-diphenylmethane diisocyanate with tripropylene glycol, (ii) liquid modified 4,4'-diphenylmethane diisocyanates containing carbodiimide groups and (iii) prepolymers from 4,4'-diphenylmethane diisocyanate and polyethers (see the first five references noted in this paragraph). U.S. Pat. No. 4,590,219 also describes the use of a prepolymer from 4,4'-diphenylmethane diisocyanate and a diethylene glycol adipate (having an OH number of 42).
Recently, our company has evaluated the use of a prepolymer derived from 4,4'-diphenylmethane diisocyanate, a polyester (having a molecular weight of 2000 and produced from adipic acid, ethylene glycol, and butanediol), and a carbodiimidized 4,4'-diphenylmethane diisocyanate. Although this particular prepolymer, when used in a RIM process with amino polyethers yields products having excellent physical properties, it still suffers from some drawbacks. Specifically, the prepolymer is generally incompatible with the amino polyethers. This results in poor mixing and the production of non-homogeneous parts. Additionally, when zinc stearate internal mold release agents are used, the hydrolytic stability of the molded part is adversely affected.